Fix 2537 spelling (ethereum#2691)

Author: rekmarks

EIPS/eip-2537.md

@@ -240,7 +240,7 @@ Discounts table as a vector of pairs `[k, discount]`:
 
 `max_discount = 174`
 
-##### Pairing operaiton
+##### Pairing operation
 
 Cost of the pairing operation is `23000*k + 115000` where `k` is a number of pairs.